The present invention relates to an electrode arrangement in a cell for manufacture of aluminum from molten salts with dimensionally stable anodes and a liquid metal product cathode.
The currently employed Hall-Heroult process for extracting aluminum from alumina dissolved in cryolite takes place at 940.degree.-1000.degree. C., while usually the electrolysis is carried out between a horizontal anode and a liquid aluminum cathode parallel to it. The oxygen separated anodically reacts with the carbon of the anode to form carbon dioxide, so that the carbon burns away. To the same extent as the linear burning away of the anode occurs, at the cathode the aluminum metal pad builds up, so that, for a suitable cell geometry, the interpolar distance remains practically constant. After the tapping of the liquid aluminum, the interpolar distance must be re-adjusted by lowering of the anodes, and furthermore consumed carbon anode blocks must be replaced at regular intervals of time. For manufacture of these anode blocks a special factory is necessary, namely the carbon plant.
Proposals have therefore been made to replace the consumable carbon anodes by dimensionally stable anodes of oxide-ceramic material, which show a whole series of advantages:
simplification of service of the cell, PA1 reduction and improved collection of the cell waste gases, PA1 independence of variations of price and quality of petroleum coke, PA1 lower total energy consumption of the process. PA1 the aluminum surface which lies opposite the active anode surface and is in direct contact with the molten electrolyte, is smaller than this active anode surface, PA1 at the carbon bottom there is formed a collecting device for the liquid metal, sub-divided by insulating material, PA1 the pools of liquid aluminum of all the sub-divisions are connected together in communication by tubes or channels, and PA1 the total of all the aluminum surfaces exposed to the melt amounts to 10-90% of the active anode surface.
These factors should result in reduced prime cost of metal.
For dimensionally stable anodes of oxide-ceramic material, as are known for example from British Pat. No. 1,433,075, whole classes of material have been described in further publications, for example spinel structures in German OS No. 24 46 314 and in Japanese published pending application 52-140411 (1977).
The multiplicity of the proposed metal oxide systems indicates that heretofore no ideal material has yet been found, which in itself satisfies the many and partly contradictory requirements of the cryolite electrolysis, while being economical.
The inventors have therefore formulated the task to produce an electrode arrangement for manufacture of aluminum from molten salts with dimensionally stable anodes, in which the stability of the anode material is further improved by special means.